Stainless ferrite-austenitic steel

ABSTRACT

A PROCESS OF PRODUCING A STAINLESS FERRITIC-AUSTENITIC STEEL HAVING HIGH RESISTANCE TO INTERCRYSTALLINE CORROSION IS DISCLOSED. THE PROCESS COMPRISES ADDING TO THE STEEL TITAINUM IN SUCH AN AMOUNT THAT THE QUOTIENT TIREST/C IS AT LEAST 4. TIREST IS THE TOTAL TITANIUM PRESENT MINUS THE QUANTITIES CONSUMED TO FORM TITANIUM NITRIDE, TITANIUM OXIDE OR OTHER TITANIUM COMPOUNDS. C IS THE TOTAL CARBON CONTENT OF THE STEEL.

United States Patent 3,717,455 STAINLESS FERRITE-AUSTENITIC STEEL NilsErik Allan Hede, Lars Ivar Hellner, and Hans Elof Johansson, Karlskoga,Sweden, assignors to Aktiebolaget Bofors, Bofors, Sweden No Drawing.Filed Feb. 16, 1970, Ser. No. 11,777 Claims priority, applicationSweden, Feb. 18, 1969, 2,228/69 Int. Cl. C22c 39/20 US. Cl. 75-128 N 1Claim ABSTRACT OF THE DISCLOSURE A process of producing a stainlessferritic-austenitic steel having high resistance to intercrystallinecorrosion is disclosed. The process comprises adding to the steeltitanium in such an amount that the quotient Ti C is at least 4. Ti isthe total titanium present minus the quantities consumed to formtitanium nitride, titanium oxide or other titanium compounds. C is thetotal carbon content of the steel.

The present invention relates to a stainless fern'ticaustenitic steel.

Applicants US. patent application 823,744 describes the manufacture of astainless ferritic-austenitic steel which is highly resistant tointercrystalline corrosion, the special characteristic features of theprocess being the addition of niobium (columbium) to a stainlessferritic austenitic steel in such quantities that the quotient Nb /Camounts to at least 8, Nb being the total niobium content minus theamount of niobium used to bind the nitrogen present in the steel toniobium nitride and C is the total carbon content of the steel.

The above patent application also discloses a number of stainlessferritic-austenitic steels of specific compositions which can be used toadvantage when practicing said process to which the reader is referredfor more detailed information on the methods used in testing theresistance of the steel to intercrystalline corrosion and the otherfactors regarding the steels known from the said patent,

A steel having the following composition has previously been disclosed:

and the remainder iron.

With regard to the rule given in the following for the content ofniobium and titanium, it should be noted that, when using conventionalmetallurgical processes, a steel of the type quoted will present anitrogen content of about 0.04-0.06% This means that the givenpercentage of nitrogen will be consumed to a very high degree in bindingthis nitrogen to titanium nitride, and that the amount of residualtitanium present in the known steel will be insufiicient to eliminatethe intercrystalline corrosion in the following process.

The process of the present invention is mainly characterized in that theniobium addition taught by patent application Ser. No. 823,744 isreplaced by a titanium addition, this change being efiected in a mannerwhereby the quotient Ti C amounts to at least 4, wherewith Ti is thetotal titanium content minus the amount of titanium consumed in bindingthe nitrogen found in the steel to titanium nitride.

It should be particularly noted that, since titanium has a much greateraffinity to oxygen than has niobium, it cannot be ignored that a certainamount of titanium will be consumed in forming titanium dioxide. Theaforementioned total quantity of titanium thus represents the titaniumcontent after subtraction of the amount of titanium consumed in formingtitanium dioxide.

Similarly to the manner described in patent application Ser. No. 823,744a number of alloys have been tested, to show the conditions when niobiumis replaced by titanium.

The compositions of the sample alloys are shown in Table 1.

TABLE L-CHEMICAL COMPOSITION OF TESTED STEELS Percentual content 0!-NOTE. Tlrest has in this instance been calculated as the total Ticontent oi the steel minus the amount of titanium needed to bind thenitrogen tound in the steel to titanium nitride. The actual Tina/C ratiois smaller, since TiOl occurs in alloys 3, 4 and 5.

for instance the values of mechanical strength of the steels and theinfluence of other alloying substances included therein.

The present invention relates to the discovery that the niobium additiontaught by US. patent application Ser. No. 823,744 can be replaced by anaddition of titanium.

The alloys disclosed in Table 1 under 1, 2 and 6 have previously beentested and the result recounted in US. patent application Ser. No.823,744.

The alloys listed in Table 1 were then tested for their ability toresist corrosion, by subjecting them to boiling 3% sodium chloridesolution saturated with silver chloride for 24 hours, after first heattreating the steel samples manner described for steel with additions ofniobium. The in accordance with Table 2. results of these tests areshown in Table 3.

TABLE 3.MECHANICAL PROPERTIES Ultimate Yield tensile Elonga- Contrac-Impact Austenltic oint, strength tion (5 tion, toughness content, Steelnumber kp. mm? kpJmmJ percent percent K", kpm. percent 57 72 27 e3 0 2547 70 34 71 20 45 52 e5 27 e0 14 4e 65 a1 70 13 40 07 69 2e 67 12 17 5s0s 23 50 0 24 The results of the test are shown in Table 2. It isevident that steel 5, which is more favourable from TABLE 2 CORROSIONDATA FOR THE ALLOYS TESTED the point of view of corrosion and which hasa higher {The tests were conducted in boiling 3% NaCl solution saturatedwith 5 rest quotient, Presents Properties of Strength whichAgGlforMhrS-l will correspond with corresponding values of the Nb- Heattreatment condition alloyed steel SIS 2324 designated 6, and with regardto impact toughness even higher values. 75 o. 1h.-eo0 o 5 75 o. lh.600o. 5 975 o. 1h.8(l0 0., 9 mihs. g hr, 'am. What 1s claimed 1s:

weight Weight weight 1. A stainless ferritic-austenitic steel having ahigh Steel loss Type 0! ss Type loss Type of resistance tointercrystalline corrosion, consisting essen- No. (percent) attack(percent) attack (percent) attack tiany of:

g i 5 5 PH Percent 12 1 1111111111: 25 Carbon 0.02-0.08

P Silicon, maximum of 0.5

112 I 11 I Manganese, maximum of 1.0

NOrE.P =spot attack; I=Intercrystalline attack.

As can be seen from Table 1, steel 1, which corre- Nickel sponds tosteel manufactured in accordance with SIS Molybdenum 2324 and to whichno niobium or titanium was added, was the victim of strongintercrystalline corrosion after titanium in such an amount that thefraction Ti /C only five minutes, which resulted in a considerable lossis atleast 4 and not greater than 20, Ti being defined in Weight Steels3 and Of which Steels 3 and 4 have as total titanium less the sum of thetitanium required relatively low additions of titanium, Show after a yfor the binding of the nitrogen present in the steel and tune of fivhours y high 105565 111 Weight Caused by the titanium used in formingany titanium oxide, and C intercrystalline corrosion. The amount oftitanium added to steel 5 was larger, which means that a quotient Ti /Cwas obtained which was considerably higher than that referring to thetotal carbon content of the steel, with the balance iron.

of the steels 3 and 4 where the uncorrected quotient 40 References CitedTi /C was 3.9 and 3.2 respectively. Steel 5 is given an UNITED STATESPATENTS uncorrected value of 20 for the Tr /C quotient but it should beparticularly noted that the values of Ti /C 3,414 11/1954 Dunn 75 128 Tgiven in Table 2 are uncorrected values and therefore 2,758,025 8/1956Clarke T much higher than the important actual values of Ti /C 33373318/1967 Lfimberg 75 128 T for the individual steels 3, 4 and 5. Steel 5with the higher 2,083,524 6/1937 Pays 75128 T content of titanium showedno intercrystallinc corrosion 3563729 2/1971 'Kovach 75 128 W and wasonly subject to spot corrosion. Special tests 3,574,002 4/1971 Haydean75-128 have shown that an actual quotient of Ti /0:4 or above whencorrected for oxidation to titanium dioxide HYLAND BIZOT Pnmary Emmetresults in a steel which is not subject to intercrystalline corrosionafter being disposed to the heat treatments disclosed in Table 2 andwhich is only liable to very 75128 T, 128 W moderate spot attack.

Tests have also been made to establish the strength properties of thesteels, the tests being conducted in the

